It isn't hard to make your own cutters. If I recall,my slitter has a 5/8 round hole. I took rectangular or square 01 steel and turned the end to fit the hole to make the wood chisels. If I wanted to make steel cutting tools,I'd just make a 5/8" holder from drill rod,with a round hole in the center to fit the corners of square HSS lathe bits. I'd put a couple of set screws in the round adapter and harden and draw it. Then,it would be easy to grind the HSS lathe tools to fit your needs.
I was also VERY lucky to find a Volstro slotting head(brand new) for $500.00 at Cabin Fever years ago. They cost,what? $3000.00? The grease in mine had never been disturbed. It slips over the spindle of my Bridgeport type mill,and has a 1" stroke,not adjustable,but handy for smaller work. It accepts standard HSS lathe tools which you grind as needed. I just save the Bridgy head for plane making. The Volstro is quicker to mount than swiveling the ram around. They are pretty rare,though,and no longer in business. I love my Volstro circular milling attachment,too. All their things were very expensive,and designed for light machining.